U–Pb dating of zircons from granitic leucosomes in migmatites of the Jiaobei Terrane, southwestern Jiao–Liao–Ji Belt, North China Craton: Constraints on the timing and nature of partial melting

Abstract

In the Jiaobei Terrane of the Jiao–Liao–Ji Belt, eastern China, layers and irregular lenses of granitic leucosomes are widely distributed within high-pressure (HP) mafic granulites, medium-pressure (MP) granulites, amphibolites, TTG gneisses, and HP and MP pelitic granulites. The layers and lenses may be parallelto or cross-cutthe foliation in the host rocks on different scales, and they show evidence of coalescence and migration in the formation of centimeter- to decimeter-scale segregations. The migmatized rocks extend over an area of at least 120 km × 75 km from Laixi, through Laiyang and Qixia, to SW Yantai. A combined study of mineral inclusions, cathodoluminescence (CL) images, and zircon U–Pb LA–ICP–MS dates provides clear evidence on the nature and timing of the partial melting in these granulite facies metamorphic rocks. Most zircons from the granitic leucosomes are either inherited crystals (igneous or detrital) with distinct overgrowths, or are simply new euhedral crystals. Both the overgrowths and the new crystals commonly exhibit perfect euhedral shapes, pronounced oscillatory zoning, and contain mineral inclusions of Qtz +Kfs + Pl +Ap. In contrast, the inherited cores are rounded or irregular in shape, and exhibit signs of dissolution. These observations suggest that the new zircons are anatectic in origin, and that they crystallized from melts derived from dehydration melting reactions and partial melting of the host granulite-facies rocks. Numerous U–Pb spot analyses of anatectic zircons reveal that the Jiaobei Terrane experienced a major episode of partial melting in the Paleoproterozoic, with crystallization of the melt occurring in two distinct periods from 1860.9 ± 2.2 to 1852.1 ± 2.8 Ma, and 1839.9 ± 2.9 to 1833.3 ± 5.3 Ma, as represented, respectively, by the magmatic cores and rims of the anatectic zircons. The first age group probably represents the initial crystallization of melts derived from partial melting within the Jiaobei Terrane, while the second age group may represent the completion of migmatization and crystallization of granitic leucosomes. This significant partial melting event was genetically associated with the Paleoproterozoic MP granulite-facies ‘hot’ stage of exhumation of the southwestern part of the Jiao–Liao–Ji Belt, and it induced extensive retrogression of the TTG gneisses and mafic and pelitic granulites, as well as the formation of regional migmatites.